Current methodology for the diagnosis of infection with Pneumocystis carinii (PC) relies on demonstration or organisms with typical morphology using special stains such as methanamine-silver, toluidine blue-O, etc. None of these stains are specific and they all must be interpreted with caution since yeast-like organisms or cellular materials can sometimes mimic the appearance of PC. Specific histologic staining using antibody directed against PC has not been possible due to the lack of appropriate antibody. One of us (J.K.) has produced monoclonal antibody preparations directed against human PC. These antibodies were used to develop a specific fluorescent antibody stain to be used to develop a specific fluorescent antibody stain to be used to aid in the diagnosis of PC pneumonia. Bronchial lavage specimens were tested in parallel by traditional toluidine blue-O stain and by a monoclonal antibody indirect immunofluorescence proceduce. A total of 126 specimens from 93 patients were stained for PC. Fortyfive of these (36%) were positive by toluidine blue O (TBO) and 43 (34%) were positive by IFA. These was a 98% agreement between both methods with no false positives and 2 false negatives by IFA. These results show that IFA using monoclonal antibodies can provide a simple, fast, yet sensitive method for diagnosing PC by microbiology laboratories.